1. Field of the Invention
The present invention relates to a torque detecting apparatus such as a torque sensor suitable to be used in power steering apparatus for an automobile.
2. Description of Related Art
A power steering apparatus for assisting the steering force of an automobile includes a torque detecting apparatus for detecting torque applied to a steering shaft through the operation of a steering wheel. A motor for assisting the steering force is driven in accordance with the torque detected by the torque detecting apparatus.
FIG. 1 is a half sectional view in the vicinity of a torque detecting coil in the conventional torque detecting apparatus used in the power steering apparatus. An input axis 1, to which a steering wheel (not shown) is fixed, is coaxially connected to an output axis 2, to which a steering mechanism (not shown) is fixed, with a torsion bar TB interposed therebetween. Around the outer periphery of the input axis 1 is fixed a detecting ring 3 made of a cylindrical magnetic substance so as to be penetrated by the input axis 1. Around the outer periphery of the output axis 2 is fixed another detecting ring 4 made of a cylindrical magnetic substance so as to be penetrated by the output axis 2.
A number of rectangular teeth 3a are formed with equal pitches in the peripheral direction therebetween around the left edge of the detecting ring 3. A number of rectangular teeth 4a are also formed with equal pitches in the peripheral direction therebetween around the right edge of the detecting ring 4 so as to correspond to the teeth 3a of detecting ring 3. A magnetic cylindrical body 5 having an inner flange is disposed around the outer surfaces of the detecting rings 3 and 4 at a position where the detecting ring 3 opposes the detecting ring 4. The cylindrical body 5 contains a torque detecting coil L1 wound on the inner surface of the cylindrical body 5 along the peripheral direction.
To one terminal of the torque detecting coil L1 is applied a reference voltage by a reference voltage generating circuit (not shown). When a reference voltage is applied, the cylindrical body 5 forms a magnetic circuit together with the detecting rings 3 and 4. Therefore, the inductance of the torque detecting coil L1 varies in accordance with the state of the magnetic circuit.
When the input axis 1 is rotated, the torsion bar TB is twisted to be supplied with rotation torque. Simultaneously, the opposing area between the teeth 3a of the detecting ring 3 and the teeth 4a of the detecting ring 4 is varied, thereby varying the inductance of the torque detecting coil L1.
At this point, when the other terminal of the torque detecting coil L1 is grounded via a resistor, the voltage at this terminal is decreased with a time constant defined by the inductance of the torque detecting coil L1 and the resistance of the resistor. Accordingly, the inductance of the torque detecting coil L1 can be detected by measuring the voltage at this terminal of the torque detecting coil L1 when a predetermined time has passed since the terminal is grounded. Further, the extent of the torsion of the torsion bar TB, i.e., the rotation torque applied to the torsion bar TB, can be also detected.
FIG. 2 is a block diagram of an exemplified configuration of the conventional torque detecting apparatus. A reference voltage Vref of a reference voltage generating circuit 10 is applied to one terminal of the torque detecting coil L1. The other terminal of the torque detecting coil L1 is connected to a sample hold circuit 14, and also to one terminal of a switching element 41 via a resistor R. The other terminal of the switching element 41 is grounded. The control terminal of the switching element 41 and the sample hold circuit 14 are supplied with the output of a pulse generating circuit 40. The output of the sample hold circuit 14 is supplied to an amplifier 15.
The operation of such a torque detecting apparatus will now be described.
The reference voltage Vref of the reference voltage generating circuit 10 is always supplied to one terminal of the torque detecting coil L1. Under this condition, when a pulse signal for switching on is supplied to the switching element 41 by the pulse generating circuit 40, the switching element 41 is on for the duration of the pulse width, thereby grounding the other terminal of the torque detecting coil L1 via the resistor R. As a result, the voltage at this grounded terminal of the torque detecting coil L1 is decreased with a time constant defined by the inductance thereof and the resistance of the resistor R. FIG. 3 shows the waveform of the voltage decreased by thus grounding the switching element 41.
At a predetermined time td (shorter than the width of the pulse signal for switching on) after the output of the pulse signal to the switching element 41, the pulse generating circuit 40 outputs a one-shot pulse (a sampling pulse) for sampling to the sample hold circuit 14. The sample hold circuit 14, in response to the one-shot pulse, samples and holds a voltage Vs under the voltage decrease as is shown in FIG. 3. The held voltage value is amplified by the amplifier 15 to be outputted as a torque signal voltage corresponding to the rotation torque. This procedure is repeated thereafter.
FIG. 4 is a block diagram showing an exemplified configuoeration of the pulse generating circuit 40. FIG. 5 is a timing chart showing the waveform of each signal in the pulse generating circuit 40. A pulse signal A (shown in FIG. 5) generated by a pulse generator 42 is supplied to the switching element 41 as the pulse signal for switching on, and is also supplied to a timer circuit 43. The timer circuit 43 counts the predetermined time td after the supply of the pulse signal A, and then outputs a pulse signal B (shown in FIG. 5) to a one-shot pulse generating circuit 44. The one-shot pulse generating circuit 44 receives the pulse signal B, and simultaneously outputs a one-shot pulse signal C (shown in FIG. 5) for sampling to the sample hold circuit 14. This procedure is repeated thereafter.
Such a conventional torque detecting apparatus, however, disadvantageously has limited reliability because it contains only one detection system from the reference voltage generating circuit up to the amplifier through the torque detecting coil, due to spatial restriction in the installation of the apparatus.